System and method for variable rate multiple access short message communications

ABSTRACT

A communication system and receiver is provided that facilitates increased message size in a communication system that supports a large number of transmitters sharing a common frequency band. The communication system facilitates increased message size by incorporating a plurality of transmit bit sets in each burst of data. The additional transmit bit sets are incorporated into a plurality of transmit codes that are generated using at least one additional spreading code that is orthogonal to the base spreading code. The plurality of transmit codes are then combined into one composite message and the composite message is spread again using another scrambling sequence. The final composite spread message is transmitted to the receiver in the appropriate message time slot, resulting in CDM/TDMA burst signal that facilitates increased message size.

TECHNICAL FIELD

The present invention generally relates to communication systems, andmore particularly relates to systems and methods for multiple accessshort message communications.

BACKGROUND

This invention relates to a multiple access communication systems and,in particular, to a communications system utilizing frequency division,code division and time division multiple access techniques for providingefficient use of frequency spectrum while supporting a large number oftransmitters sharing a common frequency band by having all transmitterssynchronized to a common timing reference.

Frequency division multiple access (FDMA) systems involve assigning eachuser a specific frequency for its respective transmission. Accordingly,a high number of users within an FDMA system requires a large frequencyband. For example, if ten users are desired within an FDMA system, tenseparate and independent frequencies would be required.

Time division multiple access (TDMA) systems involve multiple userssharing a common frequency but each user transmits at a specific timeand only for a predetermined time period. Accordingly, each TDMA userdoes not transmit continuously but only in its specific time slot.Therefore, for ten users within a TDMA system, each one would transmitonly one-tenth of the total transmission time. Furthermore, since eachuser within a TDMA system is transmitting only a portion of the time,each user will be required to transmit at high rates over a shorter timeinterval as the number of users increase. TDMA systems also require someform of synchronization between the transmitter and its respectivereceiver.

Code division multiple access (CDMA) systems involve each usertransmitting at the same time and at the same frequency. Further, CDMAsystems perform spread spectrum techniques by multiplying the transmitsequence by a pseudo-random pattern of ones and zeros of which thereceiver to receive the transmitted sequence knows. However, while CDMAsystems have a “soft-capacity” in that additional users may be addedwith only slight system degradation, such systems are not efficient whentransmitting only short bursts of data at low duty cycles.

Other systems have used CDMA and TDMA techniques to transmit bursts ofdata while efficiently using the frequency spectrum and supporting alarge number of transmitters/users. In these systems, many users of thecommunications system share a common frequency band, but each transmittheir respective message bursts of data at different and specific timesvia a TDMA technique, as defined by the system.

Unfortunately, these systems require synchronization betweentransmitters and receivers requiring a full duplex radio link and havebeen limited in the message size that can be sent to each user. Forexample, some systems have been limited to a message size of 120 bits.This message size is too short for some applications.

Hence, there exists a need for an improved communications system thatprovides efficient use of frequency spectrum while supporting a largenumber of transmitters sharing a common frequency band and simplexoperation and supports larger message sizes, while not requiringsynchronization between transmitters and receivers.

BRIEF SUMMARY

The present invention provides a communication system and receiver thatfacilitates increased message size in a communication system thatsupports a large number of transmitters sharing a common frequency band.The communication system facilitates increased message size byincorporating a plurality of transmit bit sets in each burst of data.The additional transmit bit sets are incorporated into a plurality oftransmit codes that are generated using at least one additionalspreading code that is orthogonal to the base spreading code. Theplurality of transmit codes are then combined into one composite messageand the composite message is spread again using another scramblingsequence. The final composite spread message is transmitted to thereceiver in the appropriate message time slot.

The communication system is thus implemented using code divisionmultiplexing (CDM) and time division multiple access (TDMA) techniquesfor transmitting bursts of data while efficiently using the frequencyspectrum and supporting a large number of transmitters/users.Specifically, many users of the communications system share a commonfrequency band, but each transmit their respective message bursts ofdata at different and specific times via a TDMA technique, as defined bythe system. The communication system facilitates increased message sizeby using CDM techniques that use a plurality of additional spreadingcodes that are orthogonal to the base spreading code. The CDM techniquesfacilitate the transmission of multiple transmit bit sets in each TDMAburst of data. To facilitate synchronization of the CDM/TDMA technique,each transmitter of the system is synchronized to a common timingreference thereby abating the need to maintain synchronization betweenthe transmitters and receivers of the system and the need for a fullduplex radio link.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a block diagram illustrating a communication system inaccordance with an exemplary embodiment of the present invention;

FIG. 2 is a pictorial diagram illustrating the TDMA frame structure ofthe communication system of FIG. 1;

FIG. 3 is a pictorial diagram illustrating a superframe including mframes similar to the TDMA frame of FIG. 2;

FIGS. 4A, 4B, 4C and 4D are pictorial diagrams illustrating thestructure of a TDMA burst within each defined time slot of a superframeof FIG. 3;

FIG. 5 is a pictorial diagram illustrating a more detailed format of thepreamble bits within the TDMA burst of FIG. 4;

FIG. 6A and 6B are pictorial diagram illustrating the generation of themessage bits within the TDMA burst of FIG. 4;

FIG. 7 is a block diagram illustrating the transmission of a multi-codeCDM/TDMA burst messages using a CDMA technique;

FIG. 8 is a block diagram illustrating a receiver for receivingmulti-code CDM/TDMA burst messages in accordance with an exemplaryembodiment; and

FIG. 9 is a block diagram illustrating a portion of a receiver forreceiving multi-code CDM/TDMA burst messages in accordance with anexemplary embodiment.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description.

The present invention provides a communication system and receiver thatfacilitates increased message size in a communication system thatsupports a large number of transmitters sharing a common frequency band.The communication system facilitates increased message size by usingcode division multiplexing (CDM) to incorporate a plurality of transmitbit sets in each burst of data. The additional transmit bit sets areincorporated into a plurality of transmit codes that are generated usingat least one additional spreading code this is orthogonal to the basespreading code. The use of additional spreading codes facilitates thesimultaneous transmission of additional message bits to a receiver andthus provide for increased message size.

Specifically, the message data is first encoded and interleaved intomultiple transmit bit sets, with each set including a portion of theoverall message data. The message data in the multiple transmit bit setsare then code division multiplexed into multiple transmit codes usingmultiple orthogonal spreading codes. The multiple transmit codes arecombined into one composite message and the composite message is spreadagain using another scrambling sequence. The final composite spreadmessage is transmitted to the receiver in the appropriate message timeslot. The receiver can then despread and demodulate the multipletransmit codes and extract the message from the transmission. Using themultiple transmit codes in this manner can significantly increase thenumber of bits that can be sent in each message. For example, in someimplementations it can result in an 8 to 10× increase in message length,and in other implementations with an increase in transmitted power itcan result in a 50 to 100× increase in message length.

The message data can be encoded and interleaved into multiple transmitbit sets and spread across multiple transmit codes using a variety ofdifferent techniques. In some embodiments the base transmit code is usedto carry a separate message, and in other embodiments the base transmitcode is combined with the other transmit codes to carry one largermessage. In other embodiments, the additional message data istransmitted in parallel, but not overlapping the preamble bits of thebase transmit bit set. In another embodiment, the additional messagedata overlaps the preamble bits in the base transmit bit set tofacilitate additional message length.

The communication system is implemented code division multiplexing (CDM)and time division multiple access (TDMA) techniques for transmittingbursts of data while efficiently using the frequency spectrum andsupporting a large number of transmitters/users. Furthermore, additionaltransmitters and receivers can be providing using frequency divisionmultiple access (FDMA) and code division multiple access (CDMA) tofacilitate multiple transmissions in each TDMA time slot. Specifically,many users of the communications system can share a common frequencyband, but each transmits their respective message bursts of data atdifferent and specific times via a TDMA technique, as defined by thesystem. To facilitate synchronization of the TDMA technique, eachtransmitter of the system is synchronized to a common timing referencethereby abating the need to maintain synchronization between thetransmitters and receivers of the system.

The TDMA technique of the present invention utilizes a TDMA superframestructure that is comprised of a plurality of TDMA frames whereby eachTDMA frame includes a predetermined number of specific time slots fortransmitting each burst of data. In this manner, different users can betransmitting at different rates because some users may be transmittingonce per frame, some users at once every two frames, and even some usersmay transmit at as little as once every superframe.

Each TDMA burst of data can include one or more messages spread acrossmultiple transmit codes using multiple orthogonal spreading codes. Thespread multiple transmit codes are combined into one composite messageand the composite message is spread again using another scramblingsequence by multiplying the burst of data to be transmitted by apseudo-random sequence of ones and zeroes and transmitting such spreadspectrum burst of data for reception by one or more receivers within thesystem.

Referring to FIG. 1, a block diagram illustrating a communicationssystem in accordance with the present invention is shown. The systemincludes T transmitters as denoted by reference numbers 2-6 fortransmitting bursts of data. Each of the T transmitters receives asignal from timing reference 1 for supplying each of the transmitterswith a precise time reference having time knowledge such as time of day,master frame counter, or super frame counter, and a precise timingsignal such as a 1 pulse per second (PPS) signal from a GPS system, or asimilar type signal from a LORAN system, or a frame strobe.

Such timing reference 1 allows each of the transmitters to transmitbursts of data at precise times and for precise time intervals. To thatend, each one of the transmitters 2-6 are assigned the following TDMAinformation by the communications system for specifying a transmitopportunity for each transmitter: (i) a time slot in which they areallowed to transmit, (ii) a position within the framing structure, (iii)an interval between successive transmissions, (iv) a spreading code tobe used for transmission, and (v) a frequency for transmission. Thefrequency assigned to each transmitter may be an absolute singlefrequency or it could be a alternated between a plurality of frequenciesor a sequence of frequencies to transmit at as in a hopping sequence.

Each of the T transmitters 2-6 transmit their information (bursts ofdata) to M receivers as denoted by reference numbers 8-12. It isimportant to note that each receiver does not have to receive thetransmitted signal from all transmitters. Additionally, a singlereceiver may receive the transmission from one, two or moretransmitters. Further, no synchronization between the transmitters andreceivers is needed because the transmitters are all synchronized to acommon timing reference 1, as described above.

The communication system also includes L processors as denoted byreference numbers 14-18 which are coupled to the M receivers 8-12whereby not all processors need not be coupled to all receivers.

As an application of the above described communications system,transmitters 2-6 may be provided and affixed to trucks of a specificcompany whereby the receivers would be located to provide adequatecoverage in the desired area where the trucks may roam. For example, thereceivers may be strategically positioned throughout a city for trackingthe trucks through the specific city or they may be located onsatellites for tracking the position of trucks nationwide or worldwide.In such an application, the transmitters 2-6 would transmit to thereceivers information identifying its specific location as well as otherinformation, warning messages, or the like. The receivers 8-12 willreceive such transmitted messages and processors 14-18 may be used todetermine what information has been transmitted. Further, in thisexample, the multiple processors may be status displays at variouswarehouses or different trucking companies' headquarters.

Referring to FIG. 2, a pictorial diagram illustrating the TDMA framestructure of the communications system of FIG. 1 is shown. TDMA frame 30includes a plurality of slots as denoted by slot 1 through slot S andidentified by reference number 32. In a preferred embodiment, the timeinterval for each slot (T.sub.s) is 100 milliseconds and S is 600.Accordingly, the time associated with one TDMA frame 30 (T.sub.frame) is60 seconds. In determining the time for each slot (T.sub.s),consideration is given to the amount of information desired to betransmitted per slot. Further, in determining frame time (T.sub.frame),consideration is given to the minimum time interval betweentransmissions of users.

TDMA bursts of data can be transmitted in each slot in the TDMA frame30, and each TDMA burst of data can include one or more messages spreadacross multiple transmit codes using multiple orthogonal spreadingcodes. The final composite spread message is transmitted to the receiverin the appropriate message time slot in the TDMA frame 30. The receivercan then demodulate the multiple transmit codes and extract the messagefrom the transmission.

Referring now to FIG. 3, superframe 40 is illustrated which includes Mframes similar to TDMA frame 30 of FIG. 2. As shown, superframe 40includes M frames and each having S times slots. Accordingly, a total ofM times S time slots are available for transmission per superframe.Referring back to the above example, for a time slot of 100 millisecondsand S=600, if M=10, then each superframe 40 has a duration of 10 minutesand provides a total of 6,000 time slots whereby the number of frames Min a superframe is determined by considering the maximum time intervalbetween transmissions.

Within each superframe, many users may be transmitting via the pluralityof M times S time slots with one user transmitting one or more messagesper each time slot. Moreover, each of the users may be transmitting atdifferent rates from the other users. For example, one user may transmitonce per frame (i.e., M times per superframe) while other users maytransmit once every two frames (i.e., M/2 times per superframe).Further, some users may just transmit once per superframe. Accordingly,as mentioned above, each user is assigned a slot number, a frameposition number and an interval number. In particular, the time slotnumber defines which slot within a frame the user will transmit, theframe position number defines the first frame in which the user willtransmit, and the interval number defines the number of frames betweentransmissions. As an example, if user A was assigned a slot number of 1,a frame position number of 1, and an interval number of 1, user a wouldtransmit in the time slots denoted with the letter A in superframe 40and, thus, would transmit M times per superframe. Likewise, if a user Bwas assigned the slot number of 2, a frame number of 1, and an intervalnumber of 2, user B would transmit in the time slots as denoted with theletter B in superframe 40 and, thus, would transmit M/2 times persuperframe. Similarly, if a user C was assigned a slot number of 4, aframe position number of 4, and an interval number of M (where m is thetotal number of frames per superframe), then user C would transmit inthe time slot as labeled C in superframe 40 and would transmit only onceper superframe. Accordingly, some users are transmitting every sixtyseconds (per frame), some users may be transmitting every other frame(every two minutes), while some may be transmitting once every tenminutes (once per superframe). Accordingly, the communication system ofthe present invention has the capability of assigning different usersmore transmission bandwidth based upon their needs through theabove-described superframe allocation.

Referring to FIGS. 4A, 4B, 4C and 4D, several different embodiments of aTDMA burst 50 are shown. Each TDMA burst 50 includes one or moremessages encoded into a plurality of transmit bit sets, which are thenspread into multiple transmit codes and transmitted within a definedtime slot of superframe 40. Each TDMA burst 50 includes a base transmitbit set 0 and M transmit bit sets 1-M. Each base transmit bit set 0 inTDMA burst 50 includes h bits of header as identified by block 52 whereh is selected based on the time it takes for the transmitter to settleand to reduce spectral splatter. In a preferred embodiment, three bitsof header were chosen.

Each base transmit bit set 0 in the TDMA burst 50 also includes p bitsof preamble as identified in block 54 whereby p is selected for theprobability of reception of the message and, in a preferred embodiment,p was chosen to be 32 bits.

Next, each base transmit bit set 0 in the TDMA burst 50 includes f bitsof fill as denoted by block 56 whereby the f bits of fill are used toallow some time for the receiver between the p bits of preamble and themessage bits to follow. The number of fill bits is selected consideringthe receiver processing time requirements and burst efficiency. In apreferred embodiment, f was chosen to be zero so as to maximize burstefficiency.

Each base transmit bit set 0 in the TDMA burst 50 also includes d bitsof data message as denoted in block 58. In a preferred embodiment, 332bits of message were used.

Next, each base transmit bit set 0 in the TDMA burst 50 includes t bitsof tail as denoted by block 60 which are used to allow the time for thetransmitter to turn off.

Each TDMA burst 50 is preferably shorter in duration than the timeinterval for each slot (Ts), whereby no transmission occurs for sometime interval after the transmission of the TDMA burst and before thebeginning of the next time slot. This time interval is selectedconsidering the distance between transmitters and receivers to accountfor the time it takes for a transmitted message to reach a receiver, tothe accuracy and jitter of the time reference between receivers, and topreventing overlap with other messages. In a preferred embodiment, thistime interval was selected to be 20 milliseconds.

As stated above, each base transmit bit set 0 in the TDMA burst 50 alsoincludes d bits of data message as denoted in block 58. To facilitateincreased message length, additional bits data of message data areincluded in transmit bit sets 1-M. In some embodiments the base transmitbit set 0 and the other transmit bit sets 1-M are used to carry parts ofthe same message. In other embodiments the base transmit bit set 0 andthe other transmit bit sets 1-M are designed to carry multiple messages.In a variation on both these embodiments, message data bits in transmitbit sets 1-M are limited to be concurrent with the data bits in the basemessage. In another variation on both these embodiments, message databits in transmit bit sets 1-M are concurrent with the data bits, fillbits and/or preamble bits of the base message to facilitate even greatermessage length.

In all these embodiments, the plurality of transmit bit sets 0-M areincorporated into a plurality of transmit codes that are generated usingat least one additional spreading code that is orthogonal to the basespreading code. The plurality of transmit codes are then combined intoone composite message and the composite message is spread again usinganother scrambling sequence. The final composite spread message istransmitted to the receiver in the appropriate message time slot.

In first embodiment illustrated in FIG. 4A, each transmit bit set 1-Mincludes d bits of data as identified by blocks 59 transmittedconcurrently with the d bits of data transmitted in the base transmitbit set 0. Additionally, in the embodiment illustrated in FIG. 4A thedata bits transmitted in the base transmit bit set 0 and the transmitbit sets 1-M are all part of message 1. Thus, the embodiment of FIG. 4Ais able to transmit one message with a total of [(d+1)*M] data bits.This is a significant increase over prior systems that would have beenlimited to the d bits of the base message. In some implementations, thetransmission of bits on transmit bit sets 1-M can result in an 8 to 10×increase in message length, and with an increase in transmit power, canresult in a 50 to 100× increase in message length.

In the second embodiment illustrated in FIG. 4B, each transmit bit set1-M again includes d bits of data as identified by blocks 59 transmittedconcurrently with the d bits of data transmitted in the base transmitbit set 0. In the embodiment illustrated in FIG. 4B the data bitstransmitted in transmit bit set 0 are part of message 1, and the databits transmitted in transmit bit sets 1-M are all part of message 2.Thus, the embodiment of FIG. 4B is able to transmit one message with ddata bits and one message with (d*M) data bits. Again, this is asignificant increase over prior systems that would have been limited tothe d bits in the first message.

In the third embodiment illustrated in FIG. 4C, each transmit bit set1-M includes (d+p+f) bits of message data as identified by blocks 61transmitted concurrently with the p, f and d bits of data transmitted inthe base transmit bit set 0. In the embodiment illustrated in FIG. 4Cthe data bits transmitted in transmit bit set 0 and the data bitstransmitted in transmit bit sets 1-M are all part of message 1. Thus,the embodiment of FIG. 4C is able to transmit one message with[(M*(d+p+f))+d]data bits. Because d+p+f data bits can be transmitted ineach of the transmit bit sets 1 through M, the embodiment illustrated inFIG. 4C can transmit an even larger message than those illustrated inFIGS. 4A and 4B. However, this embodiment additional buffering istypically required in the receiver as the receiver is synchronized tothe P bits of the preamble and the transmit bits 1 though M arrivebefore this synchronization is achieved.

In the fourth embodiment illustrated in FIG. 4D, each transmit bit set1-M again includes (d+p+f) bits of message data as identified by blocks61 transmitted concurrently with the p, f and d bits of data transmittedin the base transmit bit set 0. In the embodiment illustrated in FIG. 4Dthe data bits transmitted in transmit bit set 0 are part of message 1and the data bits transmitted in transmit bit sets 1-M are all part ofmessage 2. Thus, the embodiment of FIG. 4D is able to transmit onemessage with d data bits and second message with [(M*(d+p+f))] databits. Because d+p+f data bits can be transmitted in each of theorthogonal codes 1-M, the embodiment illustrated in FIG. 4D can transmitone large message is codes 1-M and one standard length message in code0.

Again, in each of the embodiments illustrated in FIGS. 4A, 4B, 4C and4D, the plurality of transmit bit sets 0-M are incorporated into aplurality of transmit codes that are generated using at least oneadditional spreading code that is orthogonal to the base spreading code.The plurality of transmit codes are then combined into one compositemessage and the composite message is spread again using anotherscrambling sequence. The final composite spread message is transmittedto the receiver in the appropriate message time slot.

In some applications it is desirable to support two different types ofreceivers. For example in a tracking system, the locations of the itemsbeing tracked is received by all receivers, but a longer messageattached may be private and desirably seen by a subset of the receivers.Further, a receiver that needs only to receive the location data,Message 1, is simpler because only one spreading code is used. Theembodiments illustrated in FIG. 4A, 4B and 4D support this capability.In the embodiment illustrated in FIG. 4C, all receivers process a singlemessage.

Referring to FIG. 5, a more detailed format of the preamble bits ofblock 54 of FIG. 4 is shown. In particular, FIG. 5 illustrates preambleblock 54 as including a signature sequence block 68 of length s and aunique word block 69 of length u bits whereby s+u=p bits. In a preferredembodiment, p=32 as mentioned and s and u=16.

The signature sequence block 68 includes s bits that are different foreach spreading code as a sequence are orthogonal, thereby providing goodcross correlation properties between the different CDMA channels.

Unique word block 69 includes u bits whereby each of the transmittersshare the same unique word thereby providing good auto correlationproperties for each CDMA channel and for improving bit synchronizationin the receiver.

As stated above, the message data is encoded and interleaved into aplurality of transmit bit sets and then is spread across multipletransmit codes. The message data can be encoded and interleaved into aplurality of transmit bit sets using a variety of different techniques.In some embodiments the base message is used to carry a separatemessage, and in other embodiments the base message is combined with theother channels to carry one message. Turning now to FIGS. 6A and 6B, twoembodiments for the generation of bit sets are illustrated. Theseembodiments are used to generate the bit sets that are spread acrossmultiple transmit codes. In FIG. 6A, an exemplary embodiment where allthe transmit bits are part of one message is illustrated. In FIG. 6B, anexemplary embodiment where one transmit bit set is used for a regularlength message 1 and the remaining transmit bit sets are used for anextended length message 2.

Specifically, FIG. 6A, is a pictorial diagram illustrating thegeneration of bit sets for one message as shown in blocks 58 and 59 ofFIG. 4A and blocks 58 and 62 of FIG. 4C. FIG. 6A illustrates that actualmessage 1 bits are transmitted along with cyclic redundancy check (CRC)bits and flush bits. In one exemplary embodiment where M=7, the messagedata bits can comprise 1306 bits, the CRC bits can comprise 16 bits, andthe flush bits can comprise 6 bits for a total of 1328 actual data bits.

These bits are sent through the encoder 74 for performing forward errorcorrection coding on the 1328 bits thereby providing for errorcorrecting capability. One skilled in the art would appreciate that anumber of encoders may be used including convolutional encoders, BCHencoders, Turbo Product Codes, Parallel Concatenated Turbo Codes, SerialConcatentated Turbo Code and Reed Solomon encoders. In one example,convolutional encoder is used with a rate of ½, and thus, 1328 actualdata bits supplied to the encoder resulted in 2656 bits to betransmitted while having the capability of error correcting. The rate ofthe convolutional encoder 74 may be chosen based upon certain conditionssuch as expected number of errors, coding gain, and data transmissionspeed. These encoded bits are sent through interleaving multiplexer 75which interleaves the order of the bits prior to transmission to improvethe performance of the error correcting capability in the presence ofburst errors. The interleaving multiplexer 75 interleaves the encodedmessage data into M+1 transmit bit sets that will be transmitted intransmit codes 0-M. For example if the embodiment illustrated in FIG. 4Ais used, with M=7, each of the transmit bit sets include ⅛^(th) of theencoded 2656 bits and is 332 bits long. Alternatively, if the embodimentillustrated in FIG. 4C is used, with P=32 and F=0, and M=7, transmit bitset 0 contains 318 bits of the encoded 2656 bits, and transmit bit sets1 through 7 contain 334 bits of the encoded 2656 bits.

Turning now to FIG. 6B, an embodiment is illustrated where some databits are for a regular length message 1 and the remaining data bits arefor an extended length message 2. Specifically, FIG. 6B, is a pictorialdiagram illustrating the generation of transmit bit sets for twomessages as shown in blocks 58 and 61 of FIG. 4B and blocks 58 and 63 ofFIG. 4D. FIG. 6B illustrates that actual message 1 and message 2 bitsare transmitted along with cyclic redundancy check (CRC) bits and flushbits. In one exemplary embodiment the message 1 data bits can comprise144 bits, the CRC1 bits can comprise 16 bits, and the flush1 bits cancomprise 6 bits for a total of 166 actual data bits. Likewise, themessage 2 data bits can comprise 1140 bits, the CRC2 bits can comprise16 bits, and the flush 2 bits can comprise 6 bits for a total of 1162actual data bits.

In this embodiment, the bits for message 1 are sent through the encoder76 for performing forward error correction coding on the message bitsthereby providing for error correcting capability. Likewise, the bitsfor message 2 are sent through encoder 79. Again, skilled in the artwould appreciate that a number of encoders may be used includingconvolutional encoders, BCH encoders, Turbo Product Codes, ParallelConcatenated Turbo Codes, Serial Concatenated Turbo Codes andReed-Solomon encoders, and that the rate of the convolutional encodermay be chosen based upon a variety of conditions.

These encoded bits for message 1 are sent through interleavingmultiplexer 77 which interleaves the order of the bits prior totransmission to improve the performance of the error correctingcapability in the presence of burst errors. The encoded bits for message2 are sent through interleaving multiplexer 80 for the same purpose.Additional, interleaving multiplexer 80 also interleaves the encodedmessage data into M transmit bit sets that will be transmitted intransmit codes 1-M. For example in an embodiment where 1162 bits aresent to the encoder and a rate ½ Turbo Product Code is used for Encoder79, 2324 encoded bits are routed to interleaving multiplexer 80. WithM=7, each transmit bit set would include 332 of these 2324 encoded bits.

It should be noted that in all these embodiments the number of flushbits would typically selected based upon the constraint length of thedecoder used in the receiver to insure that a sufficient number oftransmitted bits exists to allow for a decision to be made on the lastbit of the CRC.

As described above, the present invention provides a communicationsystem and receiver that facilitates increased message size in acommunication system that supports a large number of transmitterssharing a common frequency band. The communication system facilitatesincreased message size by incorporating a plurality of transmit bit setsinto each burst of data. The additional transmit bit sets areincorporated into a plurality of transmit codes that are generated usingat least one additional spreading code that is orthogonal to the basespreading code. The additional spreading codes facilitate thesimultaneous transmission of additional message bits to a receiver andthus provide for increased message size. Specifically, the message datais first encoded and interleaved into multiple transmit bit sets, witheach set including a portion of the overall message data. The messagedata in the multiple transmit bit sets are then code divisionmultiplexed over multiple transmit codes using multiple orthogonalspreading codes. The multiple transmit codes are combined into onecomposite message and the composite message is spread again usinganother scrambling sequence. The final composite spread message istransmitted to the receiver in the appropriate message time slot.

Referring to FIG. 7, a block diagram illustrating the how multipletransmit bit sets can be spread into multiple transmit codes, combinedinto one composite message, spread again using a scrambling sequence andtransmitted as a TDMA burst is shown. The message data is received froman interleaving multiplexer 81 that formats the messages into the M+1transmit bit sets. The interleaving multiplexer can comprise one or moreinterleaving multiplexer such interleaving multiplexers 75, 77 and 80 asillustrated in FIGS. 6A and 6B.

A TDMA burst that includes M+1 transmit bit sets is spread over M+1transmit codes using M+1 orthogonal spreading codes. The multipletransmit codes are then combined into one composite message and spreadagain using another scrambling sequence. The TDMA burst can have anysuitable structure, including those illustrated in FIG. 4A, 4B, 4C or4D. The 0-M transmit bit sets are spread to M+1 orthogonal transmitcodes using the M+1 Orthogonal Spreading generator 82. A variety oftechniques and procedures can be used to spread the message data intoM+1 orthogonal transmit codes. For example, a spreading code such as aWalsh code or Hadamard transform can be used to spread the message datato the M+1 orthogonal transmit codes. For example, with a spreadingfactor of 256, there are 256 orthogonal spreading codes, permitting M upto 255. This provides over 200 times the capacity of the prior art thatutilized a single code in the transmitter.

The spread transmit codes 1-M are then combined using the summer 83. Thesummed transmit codes 1-M are then scaled in amplitude and phase shiftedby A1 to set the relative power allocated to transmit bits 1 through Mrelative to transmit bits 0. Likewise, transmit code 0 is scaled inamplitude by A0. Transmit code 0 and transmit codes 1-M are thencombined using the summer 85. For example, in one embodiment where thereception of all bits are equally important, M=7 and A1/A0=2.646 (squareroot of 7). In another embodiment where two messages are beingtransmitted by transmit bits 0 and transmit bits 1-M respectively, andwhere message 1 is more important than message 2, with M=7, A1/A0 wouldbe set to less 2.646, providing more link margin to the receiver formessage 1 than message 2. In a preferred embodiment there is a 90 degreephase difference between A1 and A0 that reduces the degradation in thereception of transmit bits 0 due to simultaneous transmission oftransmit bits 1-M.

The resulting composite signal is spread again using an additionalscrambling sequence 85. Again, a variety of techniques and procedurescan be used to spread the composite signal. Typically, it is desirableto use a scrambling sequence that has good cross correlation properties.Examples of such scrambling sequences include Gold Codes and Kasamisequences.

The spread composite signal is then modulated using a modulator 88 thatis synchronized using a time sync 89. Several different modulationtechniques can be used such as BPSK, QPSK and PSK/OFDM. The time sync 89assures that the resulting CDM/TDMA burst is transmitted at its specifictime in the slot at the carrier frequency while its spectrum is spreadover a predetermined frequency range via scrambling sequence 87. In analternate embodiment, modulator 88 is replicated and moved prior tosummer 85, after scaling by A1 and A0 to provide for the ability to usedifferent modulation on transmit bits 0 and transmit bits 1-M.Furthermore, in one embodiment a BPSK transmission at a lower data ratemay be used on transmit bits 0 versus a 16QAM transmission at the sameor higher rate on transmit bits 1 through M to provide for even highercapacity at the expense of lower link margin for message 2.

Furthermore, it should be noted that with the use of a CDMA spectrumspreading technique, more than one of the transmitters may transmit atthe same time and at the same frequency. For example, by using 64 256bit Kasami scrambling sequences, each TDMA time slot can be usedsimultaneously by up to 64 transmitters, permitting 64 times the numberof transmitters over a classical TDMA only system. Moreover, it isunderstood that two or more different carrier frequencies may be usedfor transmitting information thereby increasing the capacity androbustness of the communications system. For example, 8 differentcarriers could be used, permitting simultaneous transmission by 8transmitters in each slot for 8 times the capacity without FDM. Thecarriers could be allocated to different classes of users or differentmessage types to provide for more reliable transmission with less selfinterference to higher priority users and messages. Thus, using thesetechniques a plurality of transmitters can each transmit a CDM/TDMAburst signal in each time slot in the superframe.

In one embodiment, a scrambling sequence is synchronized to a bit periodand, thus, the scrambling code period lasts one bit period and has adefined phase with the beginning of the bit. Further, modulator 88 maytake the form of any modulator. For example, modulator 88 may performdifferential Bi-Phase-Shift Keying (BPSK) modulation whereby the samescrambling code for the previous transmitted bit sequence is transmittedif the corresponding bit within the TDMA burst was a logic 0, while theinversion of the scrambling code for the previous transmitted bitsequence is transmitted if the corresponding bit within the TDMA burstwas a logic 1. However, it is understood that many other modulationtechniques may be used such as differential BPSK or QuadraturePhase-Shift Keying (QPSK).

A variety of receivers can be used to receive the CDM/TDMA burstsignals. As described above, CDM/TDMA bust signals facilitate increasedmessage size by incorporating at least one additional transmit bit setinto each TDMA burst of data. The additional transmit bit sets areincorporated into a plurality of transmit codes that are generated usingat least one additional spreading code that is orthogonal to the basespreading code. The plurality of transmit codes are then combined intoone composite message and the composite message is spread again usinganother scrambling sequence. The final composite spread message istransmitted to the receiver in the appropriate message time slot, andthus comprises a CDM/TDMA burst signal. The use of the additionaltransmit codes facilitate the simultaneous transmission of additionalmessage bits in the CDM/TDMA burst signal thus provide for increasedmessage size. Turning now to FIG. 8, an exemplary receiver 100 isillustrated. The receiver 100 is exemplary of the type of receiver thatcan be used to receive, despread and demodulate the CDM/TDMA burstmessages described above. Such a receiver can despread and demodulatethe message data in each of the M transmit codes and output one or morereconstructed messages. This facilitates the inclusion of larger messagesizes in a TDMA burst system.

Additionally, the receiver 100 provides the ability to receive, despreadand demodulate multiple channels (1-N) for each of the multiple codes(0-M), where each channel in the code is comprises different time delaysfor signals corresponding to that code. In the communication systemsthat use spreading sequences, the received signal cannot be guaranteedto be is not perfectly aligned, and each possible variation in alignmentis referred to as a possible chip timing of the received signal. Forexample, if there are 256 chips per bit and the receiver samples at 4samples per chip, there will be 1024 possible relative time delaysbetween the despreader and the received bit time, and the receiver willreceive, despread and demodulate N of these 1024 phases for each of theM codes. The signals from these various time delay channels aredemodulated and combined to improve the overall reliability and accuracyof the received signal. Specifically, for each of the codes 0-M thereceiver 100 despreads and demodulates all the possible chip timingdelay combinations, combines these different timing channels andextracts the message from the combined timing channels.

The receiver 100 includes a tuner 101, a multi-channel, multi-codedespreader 102, a demodulator bank 106 for each code 0-N, a multiplierbank 108 for each code 0-N, a combiner 110 for each code 0-N, a decisionblock 112 for each code 0-N, message extractor(s) 114 and a selector116. The receiver 100 can rapidly despread and demodulate relativelyshort, low duty cycle, multi-code, multi-channel TDMA burst signals. Toaccomplish this, one or more CDM/TDMA burst signals are applied to aninput of tuner 101. Tuner 101 converts the received RF signal to a moreconvenient frequency for further signal processing. For example, theinput signal applied at the input of tuner 101 may be at a carrierfrequency of 900 MHz and tuner 101 may down convert the signal to a moreappropriate frequency for further signal processing such as to a carrierfrequency near DC so that digital signal processing techniques can beused. It is noteworthy that tuner 101 may not be necessary if the inputsignal is already at a carrier frequency that is suitable for furtherprocessing.

The output of tuner 101 is applied to an input of a multi-channelmulti-code despreader 102. The multi-channel multi-code despreader 102provides a plurality of despread signals to a plurality of demodulatorbanks 106. Specifically, the multi-channel multi-code despreader 102despreads the received signal and outputs M+1 transmit code signals,with each of the M+1 transmit code signals having N channels, for atotal of (M+1)*N signals.

In addition to despreading multi-code signals, the multi-channelmulti-code despreader 102 includes a plurality of channels, for example,N, for correlating the down converted input signal by a plurality ofdespreading sequences, respectively, having different phasescorresponding to N timing offsets of the spreading code phase. Thenumber of channels is selected based upon phases with potentialcorrelation due to the chip uncertainty between the transmitter andreceiver and multipath dispersion. For example, if the transmitter andreceiver are synchronized to within 10 chips and the channel is expectedto have a dispersion of 0 to 5 chips, then the number of channels wouldbe selected to span a 15 chip code range. With M+1 codes and N channelsper code, the resultant correlation levels provide the (M+1)*N pluralityof despread signals. One skilled in the art would recognize that thereare many methods and circuits which can be used to perform thecorrelation function. These include the use of a matched filter whichcan be implemented digitally using digital circuits or software, or thefilter can be implemented using analog SAW filters or Acoustic ChargeTransport (ACT) devices. Alternatively, classical despreaders using amultiplier to multiply the incoming signal by the spreading sequencefollowed by a low pass filter, again implemented using analog or digitaltechniques can be used. Several examples of despreaders that can beadapted for use in receiver 100 can be found at U.S. Pat. No. 5,629,929,by Scott D. Blanchard et al, and entitled “Apparatus for rapidinterference cancellation and despreading of a CDMA waveform”.

Each of the M+1 transmit code signals is passed to one of the M+1demodulator banks 106 that correspond to its transmit code. Eachdemodulator bank 106 includes N demodulators, with each of the Ndemodulators corresponding to and demodulating one of the N channels. Inone embodiment, each demodulator bank 106 performs noncoherentdifferential phase shift keying (DPSK) demodulation by performing a dotproduct between correlation level spaced one bit period apart, whichessentially means that the demodulators are detecting phase changes fromone bit to the next and signal quality. However, it is understood thatother demodulators known by those skilled in the art may be used. Theseinclude coherent PSK demodulation of the signal which requires carrierrecovery, and more complex demodulators which may also includeequalization of the bit stream.

The outputs of the M+1 demodulator banks 106 each provide decisions asto whether a logic 1 or a logic 0 bit was transmitted. In a preferredembodiment, demodulator banks 106 each provide “soft” decisionsrepresenting the signal quality of its respective despread signalwhereby a digital value of −256, for example, represents a strongindication that a logic 0 was transmitted, a digital value of +255represents a strong indication that a logic 1 was transmitted, and adigital value of +1 or −1 does not provide a good indication at all asto which logical value was transmitted. Other soft decision ranges whichreduce the number of bits required to represent the soft decision mayalso be used.

The outputs of each of the M+1 demodulator banks 106 are applied to amultiplier bank 108. The N demodulator outputs for code 0 are alsoapplied to N inputs of the selector 116. The selector 116 provides aplurality of weighting signals W₁-W_(N) to the multiplier banks 108 anda message indicator signal 119 to the message extractor(s) 114. Turningnow to FIG. 9, a demodulator bank 106, multiplier bank 108 and combiner110 for code 0 and code 1 are illustrated in more detail along withselector 116. The demodulator bank 106, multiplier bank 108 and combiner110 for code 1 are exemplary of the elements that would also be includedfor codes 1-N as illustrated in FIG. 8.

As illustrated in FIG. 9, the multiplier bank 108 for code 0 receives Ndemodulated signals from the demodulator bank 106 for code 0. Likewise,the multiplier bank 108 for code 1 receives the N demodulated signalsfrom the demodulator bank 106 for code 1. The multiplier bank 108 foreach code includes a multiplier 120 for each channel 1-N. A similar setof N multipliers 120 would be included in each additional multiplierbank 108. The selector 116 controls the N multipliers 120 in eachmultiplier bank 108 with N weighting signals W₁-W_(N) that are generatedfrom the N demodulated signals for code 0.

Referring to FIGS. 8 and 9 together, the weighting signals W₁-W_(N)provided by selector 116 to the multipliers 120 in the multiplier bank108 for each code 0-M are generated in response to the N demodulatedsignals from code 0, and not from the N demodulated signals from theother codes. Thus, the selector selects and weights the demodulatedsignals for all transmit codes 0-M based only on the code 0 signals.Operation in this matter provides the performance of M+1 RAKE receivers,weighting the N-channels to maximize the received signal to noise ratio,at less complexity as only one tap weight calculator is needed for eachof the M+1 codes are subjected to the same linear distortion and thushave the same optimal weight settings.

The selector 116 processes each of the N output signals from Ndemodulators for code 0 and provides a plurality of N weighting signalsW₁-W_(N) to the second input of each multiplier 120 in each multiplierbank 108. The weighting signals W₁-W_(N), are selected to optimallycombine the N channels to maximize signal to noise ratio assisted by thefact that the P bits of preamble are known. Several techniques can beused to implement the weight calculation such as a maximum likelihoodestimation based upon the P bits of preamble, Least Mean Squaredadaptation (LMS), or recursive least square (RLS) estimation. The Nweighting signals W₁-W_(N) applied to the multipliers 120 functionallyproviding the capability of M RAKE receivers, but at reduced complexity.Thus, the weight of each signal is calculated based upon the estimatedsignal quality at the output of the N demodulators corresponding to themultipliers that are coupled to the signal weight being calculated. Aweight of zero corresponds to poor signal quality, while a high weight,such as a weight of 1, corresponds to excellent signal quality.

In the preferred embodiment, the number of multipliers 120 in eachmultiplier bank 108 with a non-zero weighted value is limited to apredetermined maximum number which is greater than zero. The number ofmultipliers is preferably selected based upon the number of expectedreceived code phases which can be estimated from the expectedtransmitter and receiver filtering and RF multi-path characteristics ofthe channel. When the maximum number of weights with value other thanzero is greater than the number of multipliers, only the largest weightsare used, and the remainder are set to zero. For example, if twomultipliers are used, and there are three weights with non-zero valuesof 0.2, 0.6 and 0.8 based upon signal quality, only two non-zero weightswould be supplied to the multipliers in the bank weight and combiners,with weights of 0.6 and 0.8. In this manner, the best signals from thedemodulator bank corresponding to codes 0-M are supplied to themultiplier banks 108 via their weighted (i.e., scaled) values asimplemented by the multiplier banks 108 whereby good quality demodulatedsignals are weighted more heavily than poor quality demodulated signalsto provide improved signal to noise ration compared to equal weighting.

In a preferred embodiment, which uses a noncoherent BPSK demodulator,the number of possible values for the weights used in the multiplierbanks 108 is reduced. In this embodiment if the signal quality estimatefrom each of demodulators in the bank of demodulators 106 for code 0exceeds a predetermined threshold thereby indicating a good signalquality of the output signal of that demodulator, then selector 116provides a weighting signal value of 1 to the respective multiplierbanks 108 corresponding to that demodulator signal. Otherwise, aweighting signal value of 0 is supplied to the corresponding multiplier,banks 108 thereby indicating a poor signal quality for that demodulator.In this manner, selector 116 determines which outputs of demodulators inthe bank of demodulators for code 0 represents the best energy signaland enables these signals to be passed to and provided to combiner 110by providing a logic 1 to the respective multiplier banks 108.

In this embodiment, multiplier banks 108 can be implemented usingswitches. If a logic 1 is applied, the switch passes the respectivesignal from demodulator, if a logic zero is applied, the switch outputsa 0 value signal. However, as discussed above, selector 116 may provideweighting signal values other than 0 or 1 based upon some othermodulation scheme or criteria. Additionally, it is understood thatselector 116 may provide more than one of the outputs of demodulators106 to the combiners 110 via selector 116 providing a logic 1 to thesecond input of more than one multiplier. In a preferred embodiment,selector 116 may provide up to the number of non-zero weighting signalvalues 1's to the multipliers thereby allowing for the best energysignals to be summed by bank weight and combiner 108.

In a preferred embodiment which uses a noncoherent BPSK demodulator, andlimits the weights to a weighting signal value of zero or one, theimplementation can further be simplified if the number of non zeroweighting signal values is set to one. In this embodiment, multipliersin the multiplier banks 108 can be replaced with a single switch whichswitches the respective demodulator output from each of the demodulatorswith an associated weighting signal value of 1 to the bank weight andcombiner 110.

In addition to providing the weighting signals W₁-W_(N), the selector116 provides a message indicator signal 119 to message extractor(s) 114for indicating that the correlation level with the P preamble bitsappearing at the output of at least one of the demodulators exceeds apredetermined threshold thereby indicating that a message is present.

Each combiner 110 receives the N scaled demodulator outputs from itscorresponding multiplier bank 108 and sums the scaled signals. Thesummed scaled signals are each then provided to a corresponding decisionblock 112. Each decision block 112 compares its summed/combined signalto a set of thresholds to make a “hard” decision of the received bitvalue and provides this decision to message extractor(s) 114.

In a preferred embodiment, decision blocks 112 provide “soft decision”bits which provide a signal quality level to extractors 114. One skilledin the art will recognize that message extractors 114 may use these“soft decisions” to improve the performance of FEC used in the message.With a BPSK signal, the hard bit decision is determined by comparing thecombined signal value with zero. A value greater than zero indicates abit value of 1, a value less than zero indicates a value of 0.Alternately, simply the sign of the decision may be utilized whereby apositive sign bit denotes a bit value of 1 while a negative sign bitdenotes a bit value of 0. One skilled in the art, would recognize thatother multi-dimensional comparators may be used for QPSK, 8PSK and QAMmodulation schemes, and that for these higher order modulation schemes,each decision would generate multiple bit values.

All signals can be either real or complex signals. For example, if aQPSK demodulator is used, then the outputs of demodulators would becomplex. To properly combine multiple demodulator outputs, weightedsignals would also be complex and multipliers would be full complexmultipliers, multiplying two complex signal inputs and generating acomplex output. For a preferred embodiment using a noncoherent BPSKdemodulator, the outputs of demodulators and weighted signals would bereal. In this embodiment, multipliers would be real multipliers,multiplying two real valued signals to generate a real valued output.

Message extractor(s) 114 performs the necessary functions to recover amessage such as de-interleaving, decoding, and a cyclic redundancy check(CRC) required to extract the information contained in the message. Asdescribed with reference to FIGS. 4 and 6, each transmitted CDM/TDMAburst can include one or more messages. In the examples illustrated inFIGS. 4A, 4C and 6A, each CDM/TDMA burst included one large message. Inthis example, a single message extractor 114 would be used to recoverthe single message for the data encoded in codes 0-M. In the examplesillustrated in FIGS. 4B, 4D and 6B, each CDM/TDMA burst includedmultiple messages. In that example, multiple message extractors 114would be used to recover the multiple messages from the data encoded incodes 0-M. In either case the message extractor 114 is used to combinedata in the codes 0-M into one or more messages that are then formattedfor delivery, performing the reverse process described for interleavingmultiplexers 75, 77 and 80. Furthermore, in one preferred embodiment,the message includes the location and status of the transmitter. In analternate embodiment, the message includes compressed speechinformation.

Thus, the receiver 100 provides the ability to receive the CDM/TDMAburst signals that facilitate increased message size by incorporating atleast one additional transmit code that is generated using a pluralityof additional spreading codes that are orthogonal to the base spreadingcode. The additional spreading codes facilitate the simultaneoustransmission of additional message bits to a receiver and thus providefor increased message size.

The present invention thus provides a communication system and receiverthat facilitates increased message size in a communication system thatsupports a large number of transmitters sharing a common frequency band.The communication system facilitates increased message size byincorporating a plurality of transmit bit sets in each burst of data.The additional transmit bit sets are incorporated into a plurality oftransmit codes that are generated using at least one additionalspreading code that is orthogonal to the base spreading code. Theplurality of transmit codes are then combined into one composite messageand the composite message is spread again using another scramblingsequence. The final composite spread message is transmitted to thereceiver in the appropriate message time slot, resulting in CDM/TDMAburst signal that facilitates increased message size.

The present invention thus provides a communication system and receiverthat facilitates increased message size in a communication system thatsupports a large number of transmitters sharing a common frequency band.The communication system facilitates increased message size byincorporating at least one additional transmit code that is generatedusing a plurality of additional spreading codes that are orthogonal tothe base spreading code. The communication system is implemented usingfrequency division multiple access (FDMA), code division multiple access(CDMA), and time division multiple access (TDMA) techniques fortransmitting code division multiplexed (CDM) bursts of data whileefficiently using the frequency spectrum and supporting a large numberof transmitters/users. Specifically, many users of the communicationssystem share a common frequency band, but each transmit their respectivemessage bursts of data at different and specific times via a TDMAtechnique, as defined by the system. For each transmission of themessage burst, multiple orthogonal codes are used to increase the sizeof the transmitted message. To facilitate synchronization of the TDMAtechnique, each transmitter of the system is synchronized to a commontiming reference thereby abating the need to maintain synchronizationbetween the transmitters and receivers of the system.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the exemplary embodiment or exemplary embodiments. Itshould be understood that various changes can be made in the functionand arrangement of elements without departing from the scope of theinvention as set forth in the appended claims and the legal equivalentsthereof.

1. A communications system utilizing code division multiple access (CDMA), time division multiple access (TDMA) and code division multiplexing (CDM) techniques for efficiently transmitting bursts of data from a plurality of users at different data rates to one or more receivers within the system, comprising: a timing reference for generating a precise timing signal and a corresponding precise time reference; a plurality of transmitters, coupled to the timing reference for providing synchronization between the plurality of transmitters, for transmitting the bursts of data at precise times and time intervals and at a predetermined frequency within a TDMA superframe wherein synchronization between said plurality of transmitters and the one or more receivers is not needed; and wherein, each of the transmitted bursts of data includes message data interleaved using an interleaving multiplexer into a plurality of transmit bit sets to increase message size in each of the bursts of data, where the plurality of transmit bit sets are incorporated into a plurality of transmit codes that are generated using at least one additional spreading code that is orthogonal to a base spreading code, combined and spread using a scrambling sequence and modulated with a carrier thereby facilitating simultaneous transmission of the plurality of transmit bit sets for each burst of data within a corresponding time slot within the TDMA superframe, and wherein the TDMA superframe includes a plurality of TDMA frames, each TDMA frame including a plurality of time slots for transmitting the bursts of data.
 2. The communication system of claim 1 wherein the plurality of orthogonal spreading codes comprise Walsh codes.
 3. The communication system of claim 1 wherein the scrambling sequence comprises a Gold code.
 4. The communication system of claim 1 wherein the scrambling sequence comprises a Kasami sequence.
 5. The communication system of claim 1 wherein each burst of data includes forward error correction data bits.
 6. The communication system of claim 1 wherein each of the bursts of data comprise a first message spread across a first transmit code in the plurality of transmit codes and a second message in other of the plurality of transmit codes.
 7. The communication system of claim 1 wherein each of the bursts of data comprise a first message spread across the plurality of transmit codes.
 8. The communication system of claim 1 wherein the plurality of transmit bit sets includes a first transmit bit set and other transmit bit sets, the first transmit bit set including: a predetermined number of header bits for allowing transmitter settling and for reducing spectral splatter; a predetermined number of preamble bits for improving a probability of reception of message bits; a first predetermined number of the message bits; a predetermined number of tail bits for allowing transmitter turn-off; and wherein the other transmit bit sets includes a second predetermined number of message bits.
 9. The communication system of claim 8 wherein the second predetermined number of message bits is equal to the first predetermined number of message bits.
 10. The communication system of claim 8 wherein the second predetermined number of message bits is equal to at least the first predetermined number of message bits plus the predetermined number of preamble bits.
 11. The communication system of claim 8 wherein the second predetermined number of message bits is equal to the first predetermined number of message bits plus the predetermined number of preamble bits plus a predetermined number of fill bits.
 12. The communication system of claim 8 wherein the first predetermined number of message bits comprises a first message and wherein the second predetermined number of message bits comprises a second message.
 13. The communication system of claim 8 wherein the first predetermined number of message bits and the second predetermined number of message bits comprises a first message.
 14. The communication system of claim 8 wherein the preamble bits include: signature sequence bits for providing good cross correlation properties between different CDMA channels; and unique word bits for providing good auto correlation properties for each CDMA channel and improving bit synchronization.
 15. The communication system of claim 1 whereby each one of the plurality of transmitters is assigned (i) at least one time slot within the plurality of TDMA frames for specifying a transmit opportunity, (ii) a position for specifying a first frame within the TDMA superframe for further specifying a transmit opportunity, and (iii) an interval for specifying a number of frames between transmission of bursts of data for further specifying a transmit opportunity.
 16. A method for transmitting bursts of data in a communications system having a plurality of transmitters and a plurality of receivers, the method comprising the steps of: synchronizing the plurality of transmitters to a common timing reference thereby abating the need to provide synchronization between the plurality of transmitters and the plurality of receivers; defining a time division multiple access (TDMA) superframe that includes a plurality of TDMA frames whereby each TDMA frame includes a plurality of time slots for transmission by the plurality of transmitters; assigning each of the plurality of transmitters a time slot within the plurality of TDMA frames for transmission; transmitting a burst of data from a transmitter in the plurality of transmitters to at least one of the plurality of receivers with a time slot assigned to the transmitter, wherein the step of transmitting from the transmitter comprises: interleaving message data into a plurality of transmit bit sets to increase message size in the burst of data; spreading the plurality of transmit bit sets into a plurality of transmit codes that are generated using at least one additional spreading code that is orthogonal to a base spreading code; combining the spread plurality of transmit bit sets and spreading the combined spread transmit bit sets using a scrambling sequence; and simultaneously transmitting the combined and spread plurality of transmit bit sets in the burst of data.
 17. The method of claim 16 wherein the plurality of orthogonal spreading codes comprise Walsh codes.
 18. The method of claim 16 wherein the scrambling sequence comprises a Gold code.
 19. The method of claim 16 wherein the scrambling sequence comprises a Kasami sequence.
 20. The method of claim 16 wherein the burst of data includes forward error correction data bits.
 21. The method of claim 16 wherein the burst of data spread comprises a first message spread across a first transmit code in the plurality of transmit codes and a second message in other of the plurality of transmit codes.
 22. The method of claim 16 wherein the burst of data comprises a first message spread across the plurality of transmit codes.
 23. The method of claim 16 wherein the plurality of transmit bit sets includes a first transmit bit set and other transmit bit sets, the first transmit bit set including: a predetermined number of header bits for allowing transmitter settling and for reducing spectral splatter; a predetermined number of preamble bits for improving a probability of reception of message bits; and a first predetermined number of the message bits; and a predetermined number of tail bits for allowing transmitter turn-off; and wherein the other transmit bit sets include a second predetermined number of message bits.
 24. The method of claim 23 wherein the second predetermined number of message bits is equal to the first predetermined number of message bits.
 25. The method of claim 23 wherein the second predetermined number of message bits is equal to at least the first predetermined number of message bits plus the predetermined number of preamble bits.
 26. The method of claim 23 wherein the second predetermined number of message bits is equal to the first predetermined number of message bits plus the predetermined number of preamble bits plus a predetermined number of fill bits.
 27. The method of claim 23 wherein the first predetermined number of message bits comprises a first message and wherein the second predetermined number of message bits comprises a second message.
 28. The method of claim 23 wherein the first predetermined number of message bits and the second predetermined number of message bits comprises a first message.
 29. The method of claim 23 wherein the preamble bits include: signature sequence bits for providing good cross correlation properties between different CDMA channels; and unique word bits for providing good auto correlation properties for each CDMA channel and improving bit synchronization.
 30. The method of claim 16 wherein the step of assigning each of the plurality of transmitters a time slot within the plurality of TDMA frames comprises assigning a position for specifying a first frame within the TDMA supethame for transmission, and assigning an interval for specifying a number of frames between successive transmissions of the bursts of data.
 31. A transmitter utilizing time division multiple access (TDMA) and code division multiplexing (CDM) techniques for efficiently transmitting bursts of data, the transmitter comprising: a timing reference for generating a precise timing signal and a corresponding precise time reference; an interleaving multiplexer, the interleaving multiplexer interleaving message data into a plurality of transmit bit sets to increase message size in a burst of data; an orthogonal spreader, the orthogonal spreader spreading the plurality of transmit bit sets across a plurality of transmit codes that are generated using at least one additional spreading code that is orthogonal to a base spreading code; a combiner, the combiner combining the spread plurality of transmit bit sets; and a scrambling sequence, the scrambling sequence scrambling the combined and spread plurality of transmit bit sets for transmission of the burst of data within a corresponding time slot within a TDMA superframe, thereby facilitating simultaneous transmission of the plurality of transmit bit sets in the burst of data, wherein the TDMA superframe includes a plurality of TDMA frames, each TDMA frame including a plurality of time slots for transmitting the bursts of data.
 32. The transmitter of claim 31 wherein the plurality of orthogonal spreading codes comprise Walsh codes.
 33. The transmitter of claim 31 wherein the scrambling sequence comprises a Gold code.
 34. The transmitter of claim 31 wherein the scrambling sequence comprises a Kasami sequence.
 35. The transmitter of claim 31 wherein each burst of data includes forward error correction data bits.
 36. The transmitter of claim 31 wherein each of the bursts of data comprise a first message spread across a first transmit code in the plurality of transmit codes and a second message in other of the plurality of transmit codes.
 37. The transmitter of claim 31 wherein each of the bursts of data comprise a first message spread across the plurality of transmit codes.
 38. The transmitter of claim 31 further comprising a modulator, the modulator modulating the plurality of transmit bit sets for transmission.
 39. The transmitter of claim 31 wherein the plurality of transmit bit sets includes a first transmit bit set and other transmit bit sets, the first transmit bit set including: a predetermined number of header bits for allowing transmitter settling and for reducing spectral splatter; a predetermined number of preamble bits for improving a probability of reception of message bits; and a first predetermined number of the message bits; and a predetermined number of tail bits for allowing transmitter turn-off; and wherein the other transmit bit sets includes a second predetermined number of message bits.
 40. The transmitter of claim 39 wherein the second predetermined number of message bits is equal to the first predetermined number of message bits.
 41. The transmitter of claim 39 wherein the second predetermined number of Message bits is equal to at least the first predetermined number of message bits plus the predetermined number of preamble bits.
 42. The transmitter of claim 39 wherein the second predetermined number of message bits is equal to the first predetermined number of message bits plus the predetermined number of preamble bits plus a predetermined number of fill bits.
 43. The transmitter of claim 39 wherein the first predetermined number of message bits comprises a first message and wherein the second predetermined number of message bits comprises a second message.
 44. The transmitter of claim 39 wherein the first predetermined number of message bits and the second predetermined number of message bits comprises a first message.
 45. The transmitter of claim 39 wherein the preamble bits include: signature sequence bits for providing good cross correlation properties between different CDMA channels; and unique word bits for providing good auto correlation properties for each CDMA channel and improving bit synchronization.
 46. The transmitter of claim 39 whereby each one of the plurality of transmitters is assigned (i) at least one time slot within the plurality of TDMA frames for specifying a transmit opportunity, (ii) a position for specifying a first frame within the TDMA superframe for further specifying a transmit opportunity, and (iii) an interval for specifying a number of frames between transmission of bursts of data for further specifying a transmit opportunity. 